1. Field of the Invention
The present invention relates to an inspection method and apparatus that are capable of detecting defects such as stains or spots on the surface of a printed matter, printed character deficiencies and the like, and more particularly to an inspection method and apparatus that is capable of detecting these defects of printed matters with high accuracy by reducing influences of positional displacement of a printed matter, warped printed matter and any other printed matters conveyed under various conditions on a conveyor line or other conveyor means particularly during they are conveyed independently of each other in a sequential order. The aforesaid defects also represent any foreign matters such as dusts mixed into the inside of a printed matter, which are visible through the surface of the printed matter when the printed matter is made of a transparent and/or opaque material with printed information thereon (e.g., a paper packet for a dose of medicine).
2. Discussion of the Background
A known method to determine the presence of a defect of a printed matter involves capturing as an inspection object an image of such as a continuously-conveyed elongated printed matter to produce an inspection image (a captured image of the surface of the printed matter), and comparing the inspection image to a reference image (a captured image of the surface of a non-defect printed matter) on a pixel-by-pixel basis, so that a defect is recognized as “present” at a pixel where the density difference is larger than a predetermined value.
In the above prior method, where positional displacement exists between the compared reference image and inspection image, accurate detection of the defect is unlikely to be made. In order to address this problem, various methods for accurate positioning both images were proposed.
Another approach to address the above problem hitherto proposed is to reduce the possibility of misjudgment due to error in image positioning by masking a specific region of a printed matter such as an edge of a printed image at which a sharp increase or decease in density difference occurs if the positional displacement has been caused, so that such a region is removed from the inspection image.
Still another approach to address the above problem hitherto proposed such as in Japanese Patent Application Laid-open No. Hei-7-186375 is to detect a defect while avoiding the occurrence of a region, which might be removed from the inspection image by the masking process, by applying a gray scale dilation filter or a gray scale erosion filter to any one of the reference and inspection images, generating a differential image and converting the same into binary data.
However, the inspection methods as described above each are intended to inspect printed matters successively conveyed under a secured condition. Therefore, these methods may not accurately inspect printed matters conveyed under unstabilized imaging condition (e.g., lighting condition), such as those conveyed independently of each other in a sequential order under various conditions, which are likely to be relatively largely displaced from the correct or intended position within the imaging field of view, or warped. Hence, the conventional methods have a problem that a minor defect of such printed matters may not be detected only by converting a differential image into binary data.
Particularly, where printed matters to be inspected are provided by punching or die cutting a substrate on which printing has been applied, punching errors must be taken into account. That is, such punching error makes it unlikely to punch or die cut the matter at a constant position in every operation, with the result that printed matters produced by punching or die cutting may have different contours and cause positional displacement on the conveyor. Thus, the positional displacement of the contour of the inspection image relative to the contour of the reference image already exists before the inspection, thus posing a problem of necessarily causing a region, which may not be inspected on the basis of the differential image.
The present invention has been conceived to address the above problems. It is an object of the present invention to provide an inspection method and apparatus that are capable of achieving inspection of a defect with high accuracy by reducing influences of printed matters, which have been displaced from the correct or intended position during the conveyance, or warped printed matters.
It is another object of the present invention to provide an inspection method and apparatus that are capable of achieving inspection of a defect with high accuracy by reducing influences of printed matters, which have contours displaced from each other, such as those produced by punching or die cutting a substrate with printing applied thereon.